Field of the Invention and Related Art Statement
The present invention relates to a circuit for producing a brightness signal from an output signal of a solid state image pick-up apparatus, in which at least two solid state image sensor each having a number of light receiving elements arranged in matrix are arranged in such a manner that light receiving elements of one solid state image sensor are shifted spatially in a main-scanning direction over a distance substantially equal to one-half of the pitch of successive light receiving elements with respect to light receiving elements of other solid state image sensor.
In a solid state image sensor, each of the light receiving elements constituting pixels are existent independently from each other and a sample signal is obtained spatially. Therefore, the maximum spatial frequency which can be reproduced by a single solid state image sensor is limited to f.sub.c /2 in accordance with the Nyquist sampling theory, wherein f.sub.c is the horizontal clock frequency. When a frequency range higher than the maximum spatial frequency f.sub.c /2 is obtained, a higher frequency component is folded back toward a lower frequency range, which results in a spurious signal. In order to obtain a higher resolution without producing the spurious signal component, in a color television camera using three solid state image sensors, light receiving elements of a green image sensor are spatially shifted in the main-scanning direction, i.e. horizontal scanning direction with respect to light receiving elements of red and blue solid state image sensors over a distance which is substantially equal to the pitch of an arrangement of the light receiving elements viewed in the horizontal scanning direction. Such a method is usually called a spatial pixel shift.
FIG. 1A is a schematic view showing a known color television camera using the above explained spatial pixel shift. As illustrated in FIG. 1A, an image of an object is formed by an objective lens 1 and is divided by a three-color splitting optical system 2 into three primary color images, i.e. red, green and blue images. These color images are made incident upon solid state image sensors 3R, 3G and 3B, respectively. As depicted in FIG. 1B, pixels of the solid state image sensor 3G receiving the green image are spatially shifted in the horizontal direction with respect to pixels of the solid state image sensors 3R and 3B for receiving the red and blue images, respectively over a distance which is equal to a half of a pitch P of the pixels viewed in the horizontal direction. By using the spatial pixel shift method, the spatial sampling is carried out such that successive pixels of the red and blue image sensors 3R and 3B are situated between successive pixels of the green image sensor 3G (FIG. 1C). When the brightness signal is formed by adding the green, red and blue signals to each other, the number of pixels is apparently increased, and thus the resolution is improved, while the spurious component folded back into the lower frequency range can be reduced.
FIG. 2 is a block diagram showing a known circuit for producing a brightness signal from the red, green and blue color signals generated by the red, green and blue image sensors 3R, 3G and 3B, respectively of the color television camera using the spatial pixel shift. The red, green and blue color signals are supplied to correlation double sampling circuits 4R, 4G and 4B, respectively, and then the green color signal is delayed by a delay circuit 5 having a delay time which is equal to one-half of the sampling period, so that the phase of the green color signal is made coincident with the phase of the red and blue color signals. It should be noted that said sampling period corresponds to the pitch of light receiving elements of the image sensors 3R, 3G and 3B viewed in the horizontal scanning direction. Then, the red, green and blue color signals having the coincided phase are supplied to low pass filters 6R, 6G and 63 which cut off frequency components higher than the sampling frequency to remove sampling clock noise. Further, the red, green and blue color signals are supplied to image signal processing circuits 7R, 7G and 7B, respectively which perform amplitude compression and .gamma. correction. Finally, the red, green and blue color signals are supplied to a matrix circuit 8 and are mixed with each other at a predetermined ratio to derive the brightness signal. In accordance with the NTSC standards; the red, green and blue color signals are added to each other at a ratio of 0.3:0.59:0.11. The derived brightness signal is amplified by an amplifier 9 and then is supplied to an output terminal of the television camera. Since the spatial pixel shift is utilized, the spurious signal folded back toward the lower frequency range which might deteriorate the quality of the reproduced color image is reduced and the resolution of the brightness signal is apparently improved.
In the known color television camera, the phase of the green color signal is delayed with respect to the red and blue color signals by passing the green color signal through the delay circuit 5 having the delay time corresponding to 1/2 P. However, the color signals are processed by the low pass filters 6R, 6G, 6B and image signal processing circuits 7R, 7G, 7B which also cause time delays in the color signals. Therefore, the phases of the red, green and blue color signals are not made identical to match each other precisely and the resolution of the brightness signal is decreased.
Further, in order to attain an improvement in the resolution by utilizing the spatial pixel shift, the circuits provided between the color image sensors 3R, 3G, 3B and the matrix circuit 8 must have frequency characteristics which can process signals having frequencies sufficiently higher than the clock frequency of the solid state image sensors.
When an object having a pattern having a repetitive pitch substantially equal to the pixel pitch as shown in FIG. 3A is picked-up by the known television camera and the solid state image sensors are read out by the clock illustrated in FIG. 3B, the green color signal is delayed with respect to the red and blue color signals by the time corresponding to a half of the pixel pitch. FIG. 3C shows the green color signal before being processed by the low pass filter 6G and FIG. 3D illustrates the red or blue color signal R or B before being transmitted through the low pass filter 6R or 6B. When these ideal color signals are mixed in the matrix circuit 8, there is derived an ideal brightness signal Y depicted in FIG. 3E. However, in practice, the low pass filters 6R, 6G and 6B have frequency characteristics such that the higher frequency component is suppressed in order to remove the clock noise. Further the image signal processing circuits 7R, 7G and 7B could hardly derive the ideal color signals shown in FIGS. 3C and 3D. Therefore, the higher frequency components of the actual green, red and blue color signals G',R' and B' obtained after being transmitted through the low pass filters 3G, 3R and 3B and image signal processing circuits 7G, 7R and 7B are suppressed as illustrated in FIGS. 3F and 3G. As a result, the frequency response of the actual brightness signal Y' is reduced to a large extent as shown in FIG. 3H.
In the spatial pixel shift, it is ideal to mix color signals at a ratio of 1:1. However, in the known color television camera using the spatial pixel shift, the green and red color signals are added to each other at a ratio of 2:1 and the green and blue color signals are mixed with each other at a ratio of 6:1. Therefore, the effect of the spatial pixel shift could not be achieved fully and the spurious signal could not be sufficiently removed and thus the resolution of the reproduced image is reduced.